An basic Blockchain example in Kotlin

basicBlockChain.kt

import com.google.gson.GsonBuilder
import java.security.MessageDigest
import java.util.Date

// This example is a port of the Java version described by:
// https://medium.com/programmers-blockchain/create-simple-blockchain-java-tutorial-from-scratch-6eeed3cb03fa

typealias BlockChain = ArrayList<Block>


fun applySha256(input: String): String {

  try {
    val digest = MessageDigest.getInstance("SHA-256")
    val hash = digest.digest(input.toByteArray())
    val hexString = StringBuffer()

    for (item in hash) {
      val hex = Integer.toHexString(0xff and item.toInt())
      if (hex.length == 1) hexString.append('0')
      hexString.append(hex)
    }

    return hexString.toString()
  } catch (e: Exception) {
    throw RuntimeException(e)
  }
}

class Block(private val data: String, val previousHash: String) {

  private val timeStamp = Date().time
  var hash: String = calculateHash()
  var nonce = 0

  fun calculateHash() = applySha256("$previousHash$timeStamp$nonce$data")

  fun mineBlock(difficulty: Int) {
    val target = "0".repeat(difficulty)

    while (hash.substring(0, difficulty) != target) {
      nonce++
      hash = calculateHash()
    }
    println("Block Mined!!! : $hash")
  }

}

fun BlockChain.isChainValid(difficulty: Int): Boolean {

  for (i in 1 until this.size) {
    val currentBlock = this[i]
    val previousBlock = this[i - 1]

    if (currentBlock.hash != currentBlock.calculateHash() ||
      currentBlock.hash.substring(0, difficulty) != "0".repeat(difficulty) ||
      previousBlock.hash != currentBlock.previousHash
    ) {
      return false
    }
  }
  return true
}


fun main() {
  val blockChain = BlockChain()

  blockChain.add(Block("Hi I'm the first block", "0"))
  blockChain[0].mineBlock(difficulty = 5)
  blockChain.add(Block("Yo I'm the second block", blockChain.last().hash))
  blockChain[1].mineBlock(difficulty = 5)
  blockChain.add(Block("Yo I'm the third block", blockChain.last().hash))
  blockChain[2].mineBlock(difficulty = 5)

  val result = GsonBuilder().setPrettyPrinting().create().toJson(blockChain)
  println(result)
  println("Blockchain is valid: ${blockChain.isChainValid(difficulty = 5)}")
}